The invention relates to a method for reducing the noise of at least two noise-affected voice channels, wherein the noise-affected voice channels are combined to create one output channel.
A method of this type is used in automatic speech recognition or in speaker phone systems to improve voice quality, for example in offices or motor vehicles.
Noise-affected speech is more easily recognizable when it is registered with two or more voice channels. Speech and noise are present in each channel. The multi-channel signals are processed with digital signal processing.
In multi-channel systems the transit time difference of the useful signal must first be determined in the individual channels. It is then possible later to recombine the individual channels in-phase into one channel.
Systems having two channels are of particular interest, because in this instance a spatial sound field can be resolved in individual directions with tolerable computing expenditure.
If it is known from which direction the relevant noise event originates, an acoustic directional lobe can be set to this event.
Noise reduction is first executed in each individual channel. Because noise reduction cannot take place error-free, distortions and artificial insertions (e.g. "musical tones") can occur. When the individually-processed channels are combined, an averaging is performed, and these errors are consequently reduced.
The composite signal is subsequently further processed with the use of cross-correlation of the signals in the individual channels. The prerequisite of this is that noises or echos are less correlated than the useful signal of the channels.
A method of combining two noise-affected voice channels is known from the publication "Multimicrophone signal-processing technique to remove room reverberation from speech signals" by Allen, Berkley and Blauert (J: Accoust. Soc. Am., Vol. 62, No. 4, October 1977) and "Noise Suppression Signal Processing Using 2-Point Received Signals" by Kaneda and Tohyame (Electronics and Communication in Japan, Vol. 67-A, No. 12, 1984). The first method is intended to remove reverberation from speech signals, and does not employ a true phase compensation; the removal of reverberation is only executed in a subsequent processing stage. The second method utilizes a simple, linear phase compensation of the channels. In this latter method, noise reduction also is executed only in the subsequent processing stage.
The object of the invention, therefore, is to provide a noise-reduction method in which noise reduction is executed in a plurality of stages and a significant improvement in speech quality is achieved.